Basic research into the mechanisms of epilepsy has provided evidence consistent with highly conserved means of altered neuronal excitability regardless of the experimental model or species involved. The more fundamental the excitability change leading to epilepsy, the more conserved across species and models its underlying mechanism should be. The present study will compare three essential aspects of hippocampal neuronal excitability in three chronic cases of temporal lobe epilepsy (TLE): l) TLE patients, 2) limbic kindling, and 3) TLE induced by kainic acid (KA). The three major cellular alterations to be compared are: 1) long-term changes in the functioning of N-methyl-D-aspartate (NMDA) receptors; 2) the degree of excitatory synaptic drive onto interneurons following supragranular sprouting of mossy fibers, and 3) compensatory alterations in neuronal calcium homeostasis. The changes in NMDA channel function will be studied in cell-attached and excised patch clamp recordings in neurons acutely isolated from human TLE and kindled hippocampi. The sensitivity of the channels to second messengers, agonists, antagonists, magnesium, and zinc will be studied by analyzing steady-state channel openings and the kinetics of channel behavior following rapid agonist applications to membrane patches. The excitatory drive onto interneurons will be measured in the KA model known for its extensive but variable degree of mossy fiber sprouting. Its magnitude will be evaluated by measuring the effect of excitatory amino acid antagonists on inhibitory postsynaptic currents recorded in the whole- cell mode in brain slices, and will be correlated with the amount of mossy fiber sprouting determined in anatomical studies. Possible similarities between the handling of calcium by human TLE and kindled neurons will be explored in whole-cell recordings of calcium currents and simultaneous calcium imaging in acutely isolated human TLE and kindled dentate gyrus granule cells. The primary goal of the proposed experiments is to unravel common cellular and molecular events underlying the altered excitability of surviving neurons in TLE. Understanding the fundamental mechanisms how neurons can sustain epileptic discharges, how they remain in this state, and how their lasting change in excitability perturbs the rest of the brain region affected by epilepsy will lead to novel therapeutical approaches aimed at restoring normal excitability in epileptogenic structures.